Blood coagulation is a complex chemical and physical reaction that occurs when blood (herein, “blood” shall mean whole blood, citrated blood, platelet concentrate or plasma, unless otherwise specified) comes into contact with an activating agent, such as an activating surface or an activating reagent. In accordance with one simplified conceptual view, the whole blood coagulation process can be generally viewed as three activities: platelet adhesion, platelet aggregation, and formation of a fibrin clot. In vivo, platelets flow through the blood vessels in an inactivated state because the blood vessel lining, the endothelium, prevents activation of platelets. When a blood vessel is damaged, however, the endothelium loses its integrity and platelets are activated by contact with tissue underlying the damaged site. Activation of the platelets causes them to become “sticky” and adhere together. Additional platelets then adhere to the activated platelets and also become activated. This process continues until a platelet “plug” is formed. This platelet plug then serves as a matrix upon which blood clotting proceeds.
If the chemical balance of the blood is suitable, thrombin is then produced that causes fibrinogen to convert to fibrin, which forms the major portion of the clot mass. During clotting, additional platelets are activated and trapped in the forming clot, contributing to clot formation. As clotting proceeds, polymerization and cross-linking of fibrin results in the permanent clot. Thus, platelet activation plays a very important function in blood coagulation.
The clinical assessment of clotting function has long been recognized to be important in the management of surgical patients. Preoperatively, the assessment of the clotting function of the patient's blood is utilized as a predictor of risk of patient bleeding, allowing advanced preparation of blood components. Perioperative monitoring of the clotting function of the patient's blood is also important because coagulopathies can be induced by hemodilution of procoagulants, fibrinogen and platelets, by consumption of coagulation factors during surgical procedures, or by cardiopulmonary bypass. Post-operative assessment of clotting function is also crucial to the patient's successful recovery. For example, 3-5% of cardiopulmonary bypass patients require surgical reoperation to stop bleeding. Prompt assessment of clotting function could rule out coagulopathy as the cause of bleeding and could avoid unnecessary surgery that adds to patient morbidity and treatment costs.
Several tests of coagulation are routinely utilized to assess the complicated cascade of events leading to blood clot formation and test for the presence of abnormalities or inhibitors of this process. Among these tests are platelet count (PLT), thrombin time, prothrombin time (PT), partial thromboplastin time (aPTT), activated clotting time (ACT), fibrinogen level (FIB) and fibrinogen degradation product concentrations. The aPTT test can also be used to assess the degree of anticoagulation resulting from heparin administration, while the PT test results can indicate the level of anticoagulation produced by warfarin administration.
During heart bypass surgery, the platelets of blood circulated in an extracorporeal circuit may become activated by contact with the materials present in the extracorporeal circuit. This activation may be reversible or irreversible. Once platelets are irreversibly activated, they lose their ability to function further. A deficiency of functional platelets in the blood may be indicative of an increased probability of a post-operative bleeding problem. Such a deficiency, and the, resulting post-operative bleeding risk, could be remedied by a transfusion of platelet concentrate. Platelet functionality tests, e.g., the ACT test, can identify a deficiency of platelets or functional platelets and aid the attending surgeon in ascertaining when to administer a platelet concentrate transfusion. Such a test is further useful in ascertaining the efficacy of a platelet transfusion. By performing the platelet functionality test following a platelet transfusion, it is possible to determine if additional platelet concentrate transfusions are indicated. Real-time assessment of clotting function at the operative site is preferred to evaluate the result of therapeutic interventions and also to test and optimize, a priori, the treatment choice and dosage.
A number of different medical apparatuses and testing methods have been developed for measuring and determining platelet activation and coagulation-related conditions of blood that can be used in real time during surgery, particularly bypass surgery, on fresh drawn blood samples or that can be used after some delay on citrated blood samples. Some of the more successful techniques of evaluating blood clotting and coagulation of fresh or citrated blood samples employ plunger techniques disclosed in commonly assigned U.S. Pat. Nos. 4,599,219, 4,752,449, 5,174,961, 5,314,826, 5,925,319, and 6,232,127, for example. These techniques are embodied in the ACT II® automatic coagulation timer, commercially sold by the assignee of this patent application.
In U.S. Pat. No. 5,302,348, an apparatus and method are disclosed for performing a coagulation time test on a sample of blood deposited in a fluid reservoir of a disposable cuvette. A capillary conduit have at least one restricted region is formed within the cuvette. The cuvette is inserted into a testing machine that engages the cuvette and draws blood from the fluid reservoir into the capillary conduit. The blood is then caused to reciprocally move back and forth within the capillary conduit so that the blood is forced to traverse the restricted region. Optical sensors of the testing machine are employed to detect movement of the blood. The testing machine measures the time required each time the blood is caused to traverse the restricted region. Coagulation is considered to have occurred and the overall coagulation time is displayed to the operator when a measured time is a predetermined percentage longer than an immediately preceding time.
In U.S. Pat. No. 5,504,011, a similar apparatus and method are disclosed for performing multiple coagulation time tests on a sample of blood deposited in a fluid reservoir of a disposable cuvette having multiple capillary conduits within the cuvette. Each of the conduits contains a dried or lyophilized activation reagent that is rehydrated by the blood. The blood in each conduit is then reciprocally moved across a restricted region of the conduit until a predetermined degree of coagulation occurs. Since the coagulation time is being monitored in multiple conduits, a representation coagulation time for a given sample can be determined. A normalizing control agent is present in at least one of the conduits. The normalizing control agent counteracts any effects of anticoagulants present in the blood sample, thereby allowing the blood sample to have generally normal coagulation characteristics. The normalized blood is tested simultaneously with the untreated blood to provide a reference value against which the functionality of the test system and the quality of the sample can be judged.
The apparatus and methods disclosed in the '348 and '011 patents only check the state of the sample during the reciprocal back and forth movement of the sample through the restricted region capillary. The detection of coagulation would be delayed or inaccurate if the sample coagulates between movement cycles.
In U.S. Pat. No. 6,200,532, a device and method for performing blood coagulation assays, particularly prothrombin times and activated partial thromboplastin times and other clotting parameters are disclosed. One embodiment of the device comprises a disposable cassette containing a sample inlet for sample delivery, a pair of interleaved spiral capillary channels for driving force, and a reaction chamber with an appropriate dry reagent for a specific assay, and a piezoelectric sensor. The device could also include a heating element for temperature control, and a magnetic bender. Compressed air is employed to drive the sample into the two spiral capillary channels. The magnetic bender is driven by an electromagnetic field generator and is attached onto a piezoelectric film in contact with the blood sample. The electric signal generated in the piezoelectric film is characterized by its frequency and amplitude due to the movement of the attached metal film. The signal collected at the site of the piezoelectric film represents the process of a biochemical reaction in the reaction chamber as the blood sample proceeds to the point at which clot formation starts and is amplified by an amplifier and rectified into a DC voltage and is sent to a recording unit and/or display unit.
Other tests can be performed on blood samples to measure coagulation that do not necessarily determine coagulation time. For example, methods and apparatus are disclosed in U.S. Pat. No. 4,780,418 for measuring the aggregation of blood platelets or the coagulation of blood drawn from a patient. The apparatus includes a capillary tube and a piston in communication with the capillary tube and connected to a motor for linear displacement thereof to draw a low pressure within the capillary tube to aspirate blood from a blood source into the capillary tube. A pressure sensor is located in the space between the drawn blood and piston, and the pressure signal is applied to a computer for comparison with a threshold. The computer calculates a motor control signal applied to the motor to regulate the pressure so that it is maintained constant in the capillary tube to mimic blood loss from a wound, for example. The computer also determines the amount of blood flowing into the capillary tube during the constant pressure regulation over a given period of time due to a known relationship relating movement of the piston to blood drawn into the cylinder. The rate of change in platelet-aggregation and blood coagulation of a patient's blood can be determined by successively employing the apparatus to draw blood samples from a patient and determine the volume drawn per unit of time.
Other blood handling equipment employ similar equipment for drawing a blood sample into a capillary tube or pipette. A system and techniques are disclosed in U.S. Pat. No. 5,540,081 for detecting clotting of a small diameter nozzle of a pipette dipped into the blood source to automatically draw a blood sample and sounding an alarm to alert an operator of the clotted pipette. The blood sample that is drawn into the pipette is apparently mixed with a reagent and tested employing other systems and techniques that are not explicitly disclosed in the '081 patent. The automatic aspiration of the required volume of blood into the pipette is complicated by partial or complete obstruction of the nozzle by incomplete or fully obstructing clots that can occur over the aspiration time. It is difficult to detect incomplete obstructing clots, and such clots can slow filling and result in incomplete filling of the pipette.
In the '081 patent, the large end of the pipette is connected to a pump through an air hose, and the pump is operated to reduce air pressure within the air hose and the pipette so that the blood sample can be aspirated into the nozzle and pipette. A pressure sensor is connected to the air hose to monitor the air pressure within the tube and develop an air pressure signal. The pressure signal is amplified by an amplifier, converted into a digital signal by an A/D converter, and applied to a plurality of pressure difference calculating circuits for comparison to a plurality of thresholds for a corresponding plurality of time periods that are selected to detect degrees of obstruction of the nozzle. An alarm circuit is included for outputting a clot detection alarm signal when at least one of said discriminating circuits discriminates that the obtained pressure difference exceeds the discrimination threshold value. The actual time that elapses between dipping the nozzle into the blood source and any clotting of the nozzle that occurs before the pipette is filled is of no importance and is not measured.
It would be desirable to provide inexpensive, relatively simple, easy to use and accurate equipment and techniques that measure one or more of the aforementioned blood coagulation times, including thrombin time, PT, aPTT, and ACT.